Heretofore, one type of conventional rotary earth boring bits having rotating cutters has been constructed from a multiplicity of arms. Typically, each arm includes a bearing pin extending from a lower end of the arm and two longitudinal surfaces extending along two sides of each arm. A cone shaped cutter is rotatably mounted on the bearing pin of each arm, and usually three arms are positioned side-by-side in a circular configuration with the longitudinal surfaces of each arm being in an abutting relationship with a mating longitudinal surface on the two remaining arms. The arms are then welded together along the abutting longitudinal surfaces. The upper end, opposite the cutters, is threaded for attachment to a drill string.
This conventional method of constructing a drill bit using three separate arms is used, at least in part, to facilitate the mounting of the rotating cutters on the bit. If the arms were welded together before the cutters were mounted, the angle of the bearing pin and the size of the cutters would make it impossible to mount three cutters on the bearing rods. However, the conventional method of construction using a multiplicity of separate arms restricts the type of steel that may be used in the construction of the drill bit. The entire arm must be made from a grade of steel having strength and hardness to meet the requirements of the bearing pin.
Additionally, this conventional construction restricts the accuracy with which the cutters may be positioned relative to one another. Even though each arm may be manufactured to very accurate tolerances, an additional positioning error is introduced when the arms are welded together, thereby compounding the error tolerances of the two processes. Also, the longitudinal weld grooves in conventional drill bits complicate the process of threading the bit for attachment to a drill string. Often, it is necessary to take preparatory measures, such as annealing before threading the conventional bit.
Thus, a need has arisen for a rotary earth boring bit that may be constructed inexpensively of a variety of steel grades. A need has also arisen for a rotary earth boring bit having a solid forged unitary body that does not have longitudinal weld grooves as found in conventional drilling bits constructed from a plurality of arms.